More correct regex for github username validation

remote.go

@@ -30,7 +30,11 @@ type remoteRepo struct {
 
 // Regexes for the different known import path flavors
 var (
-	ghRegex      = regexp.MustCompile(`^(?P<root>github\.com/([A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
+	// This regex allowed some usernames that github currently disallows. They
+	// may have allowed them in the past, so keeping it in case we need to
+	// revert.
+	//ghRegex      = regexp.MustCompile(`^(?P<root>github\.com/([A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
+	ghRegex      = regexp.MustCompile(`^(?P<root>github\.com/([A-Za-z0-9][-A-Za-z0-9]+[A-Za-z0-9]/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
 	gpinNewRegex = regexp.MustCompile(`^(?P<root>gopkg\.in/(?:([a-zA-Z0-9][-a-zA-Z0-9]+)/)?([a-zA-Z][-.a-zA-Z0-9]*)\.((?:v0|v[1-9][0-9]*)(?:\.0|\.[1-9][0-9]*){0,2}(-unstable)?)(?:\.git)?)((?:/[a-zA-Z0-9][-.a-zA-Z0-9]*)*)$`)
 	//gpinOldRegex = regexp.MustCompile(`^(?P<root>gopkg\.in/(?:([a-z0-9][-a-z0-9]+)/)?((?:v0|v[1-9][0-9]*)(?:\.0|\.[1-9][0-9]*){0,2}(-unstable)?)/([a-zA-Z][-a-zA-Z0-9]*)(?:\.git)?)((?:/[a-zA-Z][-a-zA-Z0-9]*)*)$`)
 	bbRegex = regexp.MustCompile(`^(?P<root>bitbucket\.org/(?P<bitname>[A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)

remote_test.go

@@ -67,6 +67,23 @@ func TestDeduceRemotes(t *testing.T) {
 				VCS:     []string{"git"},
 			},
 		},
+		// some invalid github username patterns
+		{
+			"github.com/-sdboyer/vsolver/foo",
+			nil,
+		},
+		{
+			"github.com/sdboyer-/vsolver/foo",
+			nil,
+		},
+		{
+			"github.com/sdbo.yer/vsolver/foo",
+			nil,
+		},
+		{
+			"github.com/sdbo_yer/vsolver/foo",
+			nil,
+		},
 		{
 			"gopkg.in/sdboyer/vsolver.v0",
 			&remoteRepo{